The present invention relates to refractory vessels and more particularly to a hemispherical dome design for a refractory liner in such a vessel.
Black liquor is a by-product of the wood pulping process. Black liquor is a mixture of hydrocarbon, caustic, chlorine and other corrosive chemicals. It is normally completely combusted in a recovery boiler. Inorganic chemicals including sodium sulfate and sodium sulfide are recovered for reuse in the pulping process. Heat produced by the complete combustion is converted to steam, which in turn is used to produce process heat and/or electrical power. An alternative device proposed for recovering inorganic chemicals from black liquor is a gasifier. In a gasifier, the black liquor is burned in a sub stoichiometric atmosphere to produce a combustible gas. Inorganic salts are recovered in the process. The combustible gases can be used directly to fuel a gas turbine, or combusted in a power boiler.
Low pressure gasification requires an insulated environment, which is obtained through a refractory lined vessel. Refractory vessels of current design for use as gasifiers employ a stainless steel jacket and a fused-cast alumina liner. The alumina liner normally has a first inner layer of blocks comprising both alpha and beta alumina and a second outer layer of blocks comprising beta alumina. A small expansion allowance is provided between the outer layer of beta alumina blocks and the stainless steel jacket.
After vessels of this design are operated for a few months, it has been found that the refractory materials react with the soda in the liquor and expand to completely consume the normal expansion allowance provided between the refractory and the stainless steel jacket. At this point, the refractory layers begin to press against the inside of the stainless steel jacket. This situation causes early failure in the refractory materials themselves and plastic deformation of the stainless steel jacket. As a consequence, refractory linings of a conventional design have been unsatisfactory for use in a black liquor gasifier.
The inventors have found that alumina refractories not only are subject to thermal expansion as is in the prior art, but are also subject to chemical expansion. Sodium in the black liquor combines with the refractory material to produce sodium aluminate. Sodium aluminate expands on the order of 130% relative to alumina. This causes not only radial expansion but expansion in the vertical direction of the refractory liner. Prior torispherical domes associated with refractories used in gasifiers required so-called skew blocks supported directly against the shell. This practice causes two problems with refractory linings that have very large expansion: a) The dome is overly constrained from expansion along the radial direction, which causes development of high stress both in the refractory and in the shell, and b) These stresses are difficult to quantify in the design of the refractory shell system. The present invention addresses these problems by utilizing a hemispherical dome with unique layers of blocks forming the hemisphere. The hemispherical dome is backed by a layer of material that has a controlled crushability that resists expansion in a measured way.
The present invention thus provides a refractory vessel including a generally cylindrical metal shell having an upper hemispherical dome. A refractory liner has a cylindrical portion spaced inwardly from the shell and a hemispherical portion spaced inwardly from the hemispherical dome. The hemispherical portion includes a plurality of circular layers of refractory bricks, each layer having a lesser diameter than the immediately preceding layer. Each layer is composed of a plurality of blocks having tops and bottoms and sides shaped to form a ring. At least one of the successive layers and the next preceding layer has blocks with interlocking keys and keyways. The keyways are preferably positioned on the next preceding layer adjacent the outer end of each of the blocks. The keys are positioned on the successive layer adjacent the outer end of the block and extend downwardly and into mating relationship with the keyways on the next preceding layer. This keyed system is required to ensure stability of the upper layers of the dome bricks, in case they do not expand as much as the lower layers (because the upper layers are not exposed to as much alkali as the lower layers).
Another feature of the hemispherical dome is that the center of curvature of the hemispherical dome comprised of the refractory is at a lower elevation than the center of curvature of the hemispherical dome comprised of the metal shell. This provides an expansion gap which increases in thickness along the curvature of the dome. This xe2x80x9ccrescent shapedxe2x80x9d gap in the dome allows for radial expansion of the dome as well as axial expansion of the cylindrical section. The entire refractory dome rises in the vertical direction as the cylindrical section expands.